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.
I'm here to discuss. To say what I think, share my experiences. That means not that I will not complain when someone claims that Moon is square. Will complain when someone jumps in without reading carefully what is written. Ah, and don't ask from me something you can self do easily - like search