macstudio external drives not mounting

all the MacOS format drives will Mount.

Did you make certain that, when those drives were new, they were ERASEd with Disk Utility? If not, they may not be MacOS format drives.

Drives like the MyBook came with software, and the manufacturer suggested you install that software.

What that software does is to "spare you the annoyance of having to re-initialize the drive" from its factory set Windows New Technology File System (NTFS, the W is silent because there are no other Operating Systems) or similar unusual (on a Mac) format. Instead, the software they provided would simulate a MacOS drive inside an NTFS file for your Mac.

The downside of using the manufacturer's software is that if the manufacturer's software is not loaded, the Macintosh Volume may not be readable, or may not be writable. This means that in startup manager, Installer, and in Recovery such as after a data loss, the Macintosh Volume may not be visible.

The standard advice given here to all users, including novice users, is to discard the manufacturers software and NEVER use it. "Best Practice" is to erase the physical device when new, using only MacOS Disk Utility, and create the fundamental data structures needed for consistent, reliable use by MacOS.

The number of drives connected all at once depends on the total current draw. I have tried to have several devices connected to all my rear ports and some will not be recognized as they overloaded the bus. I had to move the one that had the largest load to a port on my Monitor. Another solution is to get a self powered hub to add additional ports. You can go into your system report under USB and see the power consumption numbers. You may need to remove one drive at a time to determine the one that has the largest draw.

tbirdvet--

I agree completely.

My addition was just to show there is a tool available, BLUNT as it may be, that will give you some insight about how much power is available and how much is being consumed.

In addition, as you get close to the limits, do not count on that tool being perfect. You may need wider margins than reported for certain operations, such as starting up the drive.

<<. I've never used the manufacturer's software....and they've always mounted on Mac OS. >>

if true, that eliminate a whole raft of problems with the Third-party add-on NTFS Drivers not being loaded in your active macOS.

it adds weight the power supply problem as the likely issue.

In general, you will not experience a palpable slowdown on any of your drives, BUT: Ones on the same ThunderBolt port as a high-end display get less bandwidth allocated to them, and may be slightly slowed. Display data has priority, and a 4K display can use over half the bandwidth of a ThunderBolt port.

Punchline: Don't expect your FASTEST drives to maintain their absolute top speeds on the same cable as a display.

this display will tell you how much power is needed:

 Menu > About this Mac > (system report) > USB ...

thanks to this to Allan Jones

I know but it is the total draw from all (including the TB ports) that can over load the bus. Need to do one connection at a time to determine the best combination to not overload the system.

My monitor supplies power to the built in ports which takes a load off the Mac ports which is why I moved one of my drives from the Mac port to the Monitor port.