Drive boots multiple times when internal but once from USB

Hello,

I have a problem with my 2011 Mac Mini, High Sierra.

When I use my drive as an internal drive, my Mac needs to reboot multiple times (1-3 mostly), but when I use the same drive and use it as an external via USB it boots just once.

Multiple boot-ups only happen from a cold boot up. Once it goes to the login screen and I log in, turn in off, and then on again, it boot ups normally.

I have reset pram, SMC, and all that stuff.

I reinstalled my os multiple times now. Now I will try to install it fresh from the bootable drive.

Also, I have swapped the flex cable from the original to a replacement one from aliexpress. I do not have the original one anymore, so I cannot test if it is the problem.

I would really appreciate some help with this matter as I want to sell it but I do not want it to fail a new buyer.

Cheers!