Why does my StarTech USB-to-SATA dock shut off when I connect a hard drive?

First, confirm that these are bare internal SATA drives and that the dock has its own dedicated power adapter. Desktop 3.5-inch hard drives draw substantially more power during startup than portable 2.5-inch drives, especially from the 12V rail. If the dock immediately shuts down when a drive is inserted, that points more toward an overload, wiring problem, short, or defective dock than a macOS compatibility issue. Try each drive separately and inspect the SATA contacts and power adapter rating.

Removing the drives from their original enclosures wasn’t automatically a mistake, but those enclosures supplied the correct power and sometimes included electronics that made the drives work properly. A small USB hub designed for portable SSDs or 2.5-inch drives generally won’t run 3.5-inch desktop HDDs. Keeping the original enclosures, or using a properly powered 3.5-inch SATA-to-USB enclosure or docking station, is the safest approach.

Test the dock with no drive attached, then test each Seagate drive by itself. If either drive causes the dock to shut down, try a known-good power adapter with the exact voltage, connector polarity, and equal-or-higher current rating specified by StarTech. If the dock still cuts power, stop testing it with the drives and replace or return it, since repeated power cycling isn’t good for the disks.

There’s also a possible PWDIS issue with some drives that were removed from external enclosures. Certain SATA power designs use the 3.3V pins to disable a drive, and some docks don’t handle that arrangement well. However, these drives may be too old for that to be the likely explanation, and PWDIS would more commonly leave the drive powered but unavailable rather than switch off the entire dock. A different enclosure or dock that specifically supports bare 3.5-inch SATA drives would be a better test.