It is necessary that UltraEdit can set a write-lock on the file if the large file is opened without usage of a temporary file as highly recommended for large files. UltraEdit is a text editor and not a text viewer. UltraEdit expects that a user opens a text file for editing and not just for viewing.

Is the large file already write-locked by another application which periodically writes to this file?

I have 2 suggestions in case of answer is yes:

• Open the large file against recommendation with usage of a temporary file which means copying the large file by UltraEdit to local directory for temporary files.
• Open the large file using File - Open and checking option Open as read-only.
  
  Note: I have never needed this option for myself for a large file write-locked by another application and therefore I don't know if UltraEdit does not try nevertheless to write lock the large file opened read-only without usage of a temporary file.

UltraEdit < v25.10 and UEStudio < v18.10 have to convert the temporarily to UTF-16 if the large file is a UTF-8 encoded file and UltraEdit detects this (by analyzing the first 64 KB on using UltraEdit for Windows < v24.10 or UEStudio < v17.10) which of course is not possible on a read-only drive/share. See UltraEdit has started changing my large UTF-8 encoded files on open/close.

I have 2 suggestions in case of large read-only file is UTF-8 encoded:

• Open the large file against recommendation with usage of a temporary file which means copying the large file by UltraEdit to local directory for temporary files. During the copying process the file is automatically converted by UltraEdit from UTF-8 to UTF-16.
• Open the large file using File - Open and select ASCII for Open as.
  
  Note: When opening a UTF-8 encoded file as ASCII/ANSI file means no character with a code value greater decimal 127, i.e. a non ASCII character, should be inserted into file because otherwise the file is partly UTF-8 and partly ANSI encoded which is never good and breaks the rules defined by Unicode standard for Unicode encoded text. And of course non ASCII characters are displayed not correct because displayed are the UTF-8 encoding bytes of those characters in ANSI.

I have one more suggestion in case of you just want to view the large file and not edit it, i.e. search for a string and look on found occurrences or really simply view the lines:

The file manager Total Commander has a viewer named Lister built-in which can display any text file in any encoding and of any size very quickly because it is designed for just displaying file contents. It reads file contents always just in chunks and it does not matter if the file is write locked as it is impossible with the lister to edit it. The Lister is also available as freeware stand-alone application which can be downloaded from external tools page.