TrimEnd accepts a char array, not a string. This is a more explicit way to write the code:

[char[]]$charArray = @('_','D','a','t','a','.','m','d','f')

"ccra_Data.mdf".TrimEnd($charArray) #1

"ccrb_Data.mdf".TrimEnd($charArray) #2

"Project_Data.mdf".TrimEnd($charArray) #3

"Projecs_Data.mdf".TrimEnd($charArray) #4

Notice the letters I bolded above are contained in the char array.

String.TrimEnd Method