Parece que sua suspeita druidaobelix procede, apesar de ter dito que as opções da BIOS estavam em UEFI, mas tinha opção que estava UEFI and Legacy e outra com "UEFI first". Testando alterando para "only UEFI" o boot não é realizado sendo direcionado novamente para BIOS.
Outro teste que fiz foi deixar tudo apenas como Legacy, o boot roda normal para o Win10 e no caso para o Ubuntu ao invés de aparecer as opções do GNU GRUB ele entra já na interface gráfica do Ubuntu para selecionar Try ou Instal.
De qualquer forma fiz os procedimento como me instruiu:
System: Host: ubuntu Kernel: 4.4.0-31-generic x86_64 (64 bit gcc: 5.3.1)
Desktop: Unity 7.4.0 (Gtk 3.18.9-1ubuntu3.1)
Distro: Ubuntu 16.04 xenial
Machine: Mobo: Gigabyte model: H97M-Gaming 3 v: x.x
Bios: American Megatrends v: F5 date: 09/20/2015
CPU: Quad core Intel Core i5-4690 (-MCP-) cache: 6144 KB
flags: (lm nx sse sse2 sse3 sse4_1 sse4_2 ssse3 vmx) bmips: 28000
clock speeds: max: 3900 MHz 1: 3898 MHz 2: 3874 MHz 3: 3885 MHz
4: 3888 MHz
Graphics: Card-1: Intel Xeon E3-1200 v3/4th Gen Core Processor Integrated Graphics Controller
bus-ID: 00:02.0
Card-2: NVIDIA GM204 [GeForce GTX 980] bus-ID: 01:00.0
Display Server: X.Org 1.18.3 drivers: (unloaded: fbdev,vesa) FAILED: intel,nouveau
Resolution: 1920x1080@60.00hz
GLX Renderer: Gallium 0.4 on llvmpipe (LLVM 3.8, 256 bits)
GLX Version: 3.0 Mesa 11.2.0 Direct Rendering: Yes
Audio: Card-1 NVIDIA GM204 High Definition Audio Controller
driver: snd_hda_intel bus-ID: 01:00.1
Card-2 Intel Xeon E3-1200 v3/4th Gen Core Processor HD Audio Controller
driver: snd_hda_intel bus-ID: 00:03.0
Card-3 Intel 9 Series Family HD Audio Controller
driver: snd_hda_intel bus-ID: 00:1b.0
Sound: Advanced Linux Sound Architecture v: k4.4.0-31-generic
Network: Card: Qualcomm Atheros Killer E220x Gigabit Ethernet Controller
driver: alx port: d000 bus-ID: 03:00.0
IF: enp3s0 state: up speed: 1000 Mbps duplex: full
mac: fc:a:14:f7:7a:b2
Drives: HDD Total Size: NA (-)
ID-1: /dev/sda model: ST31000528AS size: 1000.2GB temp: 39C
ID-2: /dev/sdb model: Samsung_SSD_850 size: 120.0GB temp: 0C
Partition: ID-1: / size: 3.9G used: 227M (6%) fs: overlay dev: N/A
RAID: No RAID devices: /proc/mdstat, md_mod kernel module present
Sensors: System Temperatures: cpu: 29.8C mobo: 27.8C
Fan Speeds (in rpm): cpu: N/A
Info: Processes: 201 Uptime: 22 min Memory: 1071.8/7845.5MB
Init: systemd runlevel: 5 Gcc sys: 5.4.0
Client: Shell (bash 4.3.461) inxi: 2.2.35
Disk /dev/ram0: 64 MiB, 67108864 bytes, 131072 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk /dev/ram1: 64 MiB, 67108864 bytes, 131072 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk /dev/ram2: 64 MiB, 67108864 bytes, 131072 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk /dev/ram3: 64 MiB, 67108864 bytes, 131072 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk /dev/ram4: 64 MiB, 67108864 bytes, 131072 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk /dev/ram5: 64 MiB, 67108864 bytes, 131072 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk /dev/ram6: 64 MiB, 67108864 bytes, 131072 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk /dev/ram7: 64 MiB, 67108864 bytes, 131072 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk /dev/ram8: 64 MiB, 67108864 bytes, 131072 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk /dev/ram9: 64 MiB, 67108864 bytes, 131072 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk /dev/ram10: 64 MiB, 67108864 bytes, 131072 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk /dev/ram11: 64 MiB, 67108864 bytes, 131072 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk /dev/ram12: 64 MiB, 67108864 bytes, 131072 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk /dev/ram13: 64 MiB, 67108864 bytes, 131072 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk /dev/ram14: 64 MiB, 67108864 bytes, 131072 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk /dev/ram15: 64 MiB, 67108864 bytes, 131072 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk /dev/loop0: 1.4 GiB, 1459982336 bytes, 2851528 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk /dev/sda: 931.5 GiB, 1000204886016 bytes, 1953525168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x1e76e07b
Device Boot Start End Sectors Size Id Type
/dev/sda1 206848 1869635583 1869428736 891.4G 7 HPFS/NTFS/exFAT
Disk /dev/sdb: 111.8 GiB, 120034123776 bytes, 234441648 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x9979ae63
Device Boot Start End Sectors Size Id Type
/dev/sdb1 * 718848 233515007 232796160 111G 7 HPFS/NTFS/exFAT
/dev/sdb2 233517056 234438655 921600 450M 27 Hidden NTFS WinRE
Model: ATA ST31000528AS (scsi)
Disk /dev/sda: 1000GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags:
Number Start End Size Type File system Flags
1 106MB 957GB 957GB primary ntfs
Model: ATA Samsung SSD 850 (scsi)
Disk /dev/sdb: 120GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags:
Number Start End Size Type File system Flags
1 368MB 120GB 119GB primary ntfs boot
2 120GB 120GB 472MB primary ntfs diag
Warning: Unable to open /dev/sr0 read-write (Read-only file system). /dev/sr0
has been opened read-only.
Model: TSSTcorp CDDVDW SH-S223B (scsi)
Disk /dev/sr0: 1513MB
Sector size (logical/physical): 2048B/2048B
Partition Table: mac
Disk Flags:
Number Start End Size File system Name Flags
1 2048B 6143B 4096B Apple
2 1499MB 1501MB 2425kB EFI
EFI boot on HDD
Só para constar, eu voltei as configurações da BIOS para UEFI and Legacy e UEFI first.
Eu usava apenas HD com Win7 ou 8.1, não me lembro, quando consegui o SSD fiz instalação limpa do Win10 no SSD, mas não lembro se cheguei a formatar o HD, onde fica os meus arquivos. Provavelmente sim, para tirar o Sistema antigo, seria porco eu apenas deletar a pasta do Win. Sorte que na época o volume de arquivos não era tão grande então eu conseguia jogar para o SSD e formatar o HD para devolver ao HD. Hoje se por causa de tá usando Legacy tiver que limpar tanto SSD como HD vai ser bem complicado, só em último caso. Particularmente com Win10 no SSD o boot é bem rápido perto do que era antes, não sei até que ponto vale o trabalho para migrar tudo para UEFI.
Obrigado pela ajuda também selvaking