iPad suddenly not reading files on external ssd

You are very welcome.

Connecting the SSD to the Mac - and running the first aid utility - corrected any filesystem corruption. To avoid future recurrence when using the SSD with your iPad, prior to disconnecting the SSD, simply shut-down the iPad from settings:

Settings > General > Shut Down

Allow 10-15 seconds to elapse after the screen switches-off to allow the shut-down to complete prior to disconnection.

You may have encountered drive corruption due to incorrect dismount of your SSD.

The type of flash storage device that you are using will have significant impact on “safe dismount” from your iPad. Simple flash storage devices can be dismounted provided that an active write isn’t occurring. More complex devices, such as your Samsung SSD, are a completely different proposition…

The technical cause is likely to be write-caching - this being a feature of the USB Storage (in this case SSD) device.

For most desktop computers, it is possible to configure an externally attached storage device either for performance (i.e., uses write-caching) or for Quick Removal. This determines how and when information is “written” to non-volatile storage.

When configured for Quick Removal, information from the host computer is written immediately to the storage medium. This has the advantage that you may disconnect the storage device from the host computer, without risk of corrupting the file-system, at any time that data is not being actively transmitted to the storage device. Whilst being a completely valid and useful method of managing data storage, for reasons beyond the immediate scope of this description, this method is less efficient and slower.

Alternatively, when write-caching is enabled, information from the host computer is “cached” in separate (high performance) volatile storage within the storage device - and when sufficient information has been received, a “block” of data is written in single write-cycle. Whilst being faster and more efficient, this method comes with the penalty of the host computer having to notify the storage device of a impending device-disconnection before it is disconnected. This warning ensures that the storage device will flush any pending data from its volatile cache-memory to non-volatile storage - and in so doing sets a “clean switch” on the flash filesystem and signals to the computer that it is safe to disconnect.

A drive configured for write-caching, upon connection to a host computer, is checked for the “clean switch”; if present, the storage device is “mounted” and made available to the operating system. By contrast, if the “clean switch” is not detected (this will occur if the storage device was disconnected prior to being notified of a “dismount”), the filesystem must be assumed to be potentially corrupt; pending data within the write-cache may not have been written to the drive.

So, the relevance of this to iPad is simple. While iPad can be used with SSD storage, it does not fully support devices that implement write-caching. iPadOS lacks the ability to inform the storage device of imminent disconnection of the storage device from the USB bus. iPad also lacks capability to detect and scan/repair corrupt storage devices.

In summary, now that you [hopefully] understand both the cause and effect, you will now understand that you can only reliably use USB storage devices that can be configured for Quick Disconnect operation - this having major significance to non-corruption of the filesystem and reduced data throughput.

The only “safe” dismount for an SSD that employs write-caching is to fully shut down the iPad before disconnecting the drive.

If you have access to a Windows PC or Mac computer, connecting the drive to the computer might successfully reset the SSD's clean-switch - and if required, "repair" the filesystem.