Answer
The automatic refresh of a linked Smart Object in Photoshop CC 2021 occurs only when the operating system reports a file‑system change for the source file and the preference “Update Modified Smart Objects” is enabled.
Confirmed facts
- Photoshop relies on the host OS’s file‑change notification mechanism:
ReadDirectoryChangesW on Windows and the FSEvents framework on macOS.
- The “Update Modified Smart Objects” option (Edit → Preferences → File Handling) must be checked; otherwise Photoshop ignores OS notifications and requires a manual refresh.
- When the source file resides on a local NTFS (Windows) or APFS/HFS+ (macOS) volume with proper read/write permissions, a save operation in any standard application generates the expected notification and Photoshop updates the Smart Object automatically.
- On network drives (SMB, AFP, NFS) or when the file is edited via applications that bypass normal save calls (e.g., some command‑line tools or certain third‑party editors), the OS may not emit a notification that Photoshop can detect, so automatic refresh fails and a manual
Layer → Smart Objects → Update Modified Content is needed.
Likely explanation for the observed OS‑specific behavior
On Windows, the ReadDirectoryChangesW API is generally reliable for local disks, which explains the consistent behavior reported by Windows users. On macOS, FSEvents can be delayed or dropped under certain conditions (e.g., heavy I/O load, specific network mount configurations), leading to the intermittent automatic updates some macOS users notice when the file is open in another program. Neither platform guarantees background monitoring for unsupported locations or non‑standard edit workflows.
Steps to verify the behavior on your system
- Open Edit → Preferences → File Handling and ensure “Update Modified Smart Objects” is checked.
- Create a new PSD, place a linked Smart Object that points to a file stored on your local internal drive.
- Edit the source file in any standard image editor (e.g., Photoshop itself, Preview, Paint) and save.
- Observe whether the Smart Object in the PSD updates without issuing a manual refresh command.
- Repeat the test with the source file moved to a network share or edited via a command‑line tool that does not use the standard save dialog; note if auto‑refresh fails.
Missing diagnostic detail: Confirm whether the source file is located on a local volume or a network mount, as this determines whether the OS notification path is expected to work. If the file is on a network share, automatic refresh may not be reliable regardless of OS.
Outlook for future releases
Adobe has not announced a dedicated background‑file‑monitoring service for Smart Objects beyond the existing OS‑notification approach. Until such a feature is added, reliability depends on the OS’s ability to deliver change notifications and the “Update Modified Smart Objects” preference being enabled.
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