Some storage devices (e.g., some solid-state drives, including USB flash drives) use so-called 'trim' operation to mark drive sectors as free e.g. when a file is deleted. Consequently, such sectors may contain unencrypted zeroes or other undefined data (unencrypted) even if they are located within a part of the drive that is encrypted by VeraCrypt. VeraCrypt does not block the trim operation on partitions that are within the key scope of system encryption (unless a hidden operating system is running) and under Linux on all volumes that use the Linux native kernel cryptographic services. In those cases, the adversary will be able to tell which sectors contain free space (and may be able to use this information for further analysis and attacks) and plausible deniability may be negatively affected.

https://www.veracrypt.fr/en/Trim Operation.html