Studio Thunderbolt port(s) issue - Apple Thunderbolt Adaptor not working

You seem to have verified that

• The display is a 27" Thunderbolt Display that requires Thunderbolt input – and not the similar 27" LED Cinema Display. (The latter would have a third "head" on the hydra cable and would not have the FireWire 800 port or the Gigabit Ethernet port.)
• You are using the correct adapter (the Apple Thunderbolt 3-to-2 adapter). USB-C to Mini DIsplayPort adapters would not work for this display.
• You are plugging the adapter into a rear-panel Thunderbolt 4 port on a Mac Studio - a port which should easily be able to support this display (unless you have a wall of displays you aren't telling us about, and this is the +1).

You also say that this monitor works with older Macs.

I'm going to take a stab in the dark and guess that the cable permanently attached to the Thunderbolt Display is a somewhat flaky one. Not bad enough to make Macs running older versions of macOS want to cut the video signal to that monitor, but bad enough to make a Mac Studio running a recent version of macOS want to do so.

In other threads, Mr. Bennet-Alder has said that recent versions of macOS are intolerant of transmission errors (as in, they don't want to see any, and will reduce resolution or cut the video signal when they do).

If that is what Is going on, you might be able to salvage the display by getting a high-quality Thunderbolt 1/2 cable that is as short as reasonably possible, and running it between the adapter and the TB daisy-chaining port on your display. (Leaving the hydra cable unconnected.)

I don't know who first suggested this, but I have seen people posting it several times as a workaround for saving a Thunderbolt Display with a bad hydra cable. It might work just as well for one whose hydra cable is merely a little flaky.

The problem with Pegasus enclosures is likely to be COMPLETLY different. In that case, the drivers shipped inside MacOS versions when those enclosures were originally shipped have been thrown out, and are no longer included in MacOS for not being modern enough.

You may have to side-load drivers for those enclosures from the maker.

The Mac does not rely on windows-like side-loaded "Drivers" which are actually packages of resolutions and settings for a specific display. Instead, it goes straight to the immutable source -- it asks the display itself.

To get a Mac display to become active, you need the Mac to query the display, and the display to answer with its name and capabilities. Otherwise, the display will not be shown as present, and no data will be sent to the display. "No signal detected" is generated by the DISPLAY, not by the Mac.

This query is only sent at certain times:

• at startup

• at wake from sleep — so momentarily sleeping and waking your Mac may work

• at insertion of the Mac-end of the display-cable, provided everything on that cable is ready-to-go

• hold the Option key while you click on the (Detect Display) button that will appear in Displays preferences (from another display)

so try doing some of those things and see if the display comes alive.

Only the ports on the BACK of most Mac Studio models are Thunderbolt. The front ports are only 10 Gbits/sec USB-C, so they will NOT support genuine Thunderbolt devices.