Atari TT has full SCSI hard disk controller, where it works with regular SCSI commands. Mega STE has ACSI port and limited ACSI-SCSI adapter, where ACSI commands are used. So, it will load MBR of attached SCSI disk, but after that it tries with SCSI commands, what fails on Mega STE. Additionally, internal Mega STE adapter works only with disks where parity can be disabled.
You should look for some old SCSI drive suitable for Mega STE. So under 1GB, parity disable option. That should cost just few bucks.
I have mod for Mega STE internal adapter: http://atari.8bitchip.info/modmste1.html
And actually have modded adapter for sale too.
English language is like bad boss on workplace: it expecting from you to strictly follow all, numerous rules, but self bending rules as much likes