I'm unable to install Ubuntu 20.04 on a 120GiB SSD because the device is not included in the list of devices available for install. The SSD does show in gparted, with no partitions allocated, and an msdos partition table. The system is BIOS, not UEFI.
Subsequently, I've formatted the SSD with ext4, and can write to it - so there's no problem with the SSD itself.
What would prevent Ubuntu installer from seeing the SSD as a device it can install to?
Should have looked in syslog first...
The BIOS was not configured for RAID, but the SSD had previously been used in a RAID configuration (so is the RAID information stored on the disk??? where???) The fix was to configure the BIOS for RAID, delete the RAID configuration, then set the BIOS back to non-RAID. Then I was able to install to the SSD.
Another lesson learned was to disconnect all (5) unnecessary drives during install, because the install syslog is filled with irrelevant information about them - making is much harder to spot the one, crucial error message.