Thanks to hdparm -B1 /dev/sdb my HDD does no longer spin up when powered up on boot. But after completing the BIOS POST messages and starting Ubuntu the HDD gets a signal over the SATA data cable and spins up.
Leaving the data cable (but still with plugged in SATA power cable) let the system boot up completely from my SSD without spinning up the HDD.
What causes the HDD to spin up? Maybe Grub2?
Edit: nope, doesn't seem to be Grub2 that spins up the drive. I just set up Grub to show its menu without timer. Nothings happens until I hit the Ubuntu standard boot option, then a few seconds later the drive spins up.
dmesg | grep sdb:
[ 7.080043] ata2.00: qc timeout (cmd 0xef) [ 7.080057] ata2.00: failed to IDENTIFY (SPINUP failed, err_mask=0x4) [ 9.830035] ata2: SATA link up 3.0 Gbps (SStatus 123 SControl 300) [ 9.836555] ata2.00: ATA-8: SAMSUNG HD105SI, 1AJ10001, max UDMA/133 [ 9.836564] ata2.00: 1953525168 sectors, multi 0: LBA48 NCQ (depth 31/32) [ 9.842364] ata2.00: configured for UDMA/133 [ 9.842644] scsi 1:0:0:0: Direct-Access ATA SAMSUNG HD105SI 1AJ1 PQ: 0 ANSI: 5 [ 9.843249] sd 1:0:0:0: [sdb] 1953525168 512-byte logical blocks: (1.00 TB/931 GiB) [ 9.843270] sd 1:0:0:0: Attached scsi generic sg1 type 0 [ 9.843487] sd 1:0:0:0: [sdb] Write Protect is off [ 9.843497] sd 1:0:0:0: [sdb] Mode Sense: 00 3a 00 00 [ 9.843590] sd 1:0:0:0: [sdb] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA [ 9.844091] sdb: [ 9.845127] scsi 2:0:0:0: CD-ROM ATAPI iHDS118 5 RL0C PQ: 0 ANSI: 5 [ 9.851182] sr0: scsi3-mmc drive: 48x/48x cd/rw xa/form2 cdda tray [ 9.851191] Uniform CD-ROM driver Revision: 3.20 [ 9.851554] sr 2:0:0:0: Attached scsi CD-ROM sr0 [ 9.851802] sr 2:0:0:0: Attached scsi generic sg2 type 5 [ 9.942896] sdb1 [ 9.943773] sd 1:0:0:0: [sdb] Attached SCSI disk