ok so i messed up with the FileInfo, because it was already there and I just lazily used it. But lets try handling this with the sophistication it calls for

BizHawk.Client.EmuHawk/MainForm.cs

@@ -3790,26 +3790,9 @@ namespace BizHawk.Client.EmuHawk
 				file.Directory.Create();
 			}
 
-
 			// Make backup first
-			if (Global.Config.BackupSavestates && file.Exists)
-			{
-				var backup = path + ".bak";
-				var backupFile = new FileInfo(backup);
-				if (backupFile.Exists)
-				{
-					backupFile.Delete();
-				}
-
-				try
-				{
-					File.Move(path, backup);
-				}
-				catch
-				{
-					// Eat it, this will happen rarely and the user will rarely need the file, so the odds of simply not making the backup is very unlikely
-				}
-			}
+			if (Global.Config.BackupSavestates)
+				BizHawk.Common.Util.TryMoveBackupFile(path, path + ".bak");
 
 			SaveState(path, quickSlotName, false);
 

BizHawk.Common/Util.cs

@@ -24,6 +24,51 @@ namespace BizHawk.Common
 			}
 		}
 
+		/// <summary>
+		/// Tries to moves `pathWant` out of the way to `pathBackup`, delaying as needed to accomodate filesystem being sucky.
+		/// `pathWant` might not be removed after all, in case it's snagged by something.
+		/// </summary>
+		public static bool TryMoveBackupFile(string pathWant, string pathBackup)
+		{
+			//If the path we want is available we dont actually have to make a backup
+			if (!File.Exists(pathWant))
+				return true;
+
+			//delete any existing backup
+			try
+			{
+				if (File.Exists(pathBackup))
+					File.Delete(pathBackup);
+			}
+			catch
+			{
+				//just give up on the whole thing in case of exceptions. pathWant will get overwritten by the caller.
+				return false;
+			}
+
+			//deletes are asynchronous, need to wait for it to be gone
+			while (File.Exists(pathBackup))
+				System.Threading.Thread.Sleep(10);
+
+			try
+			{
+				//actually move pathWant out of the way to pathBackup now that pathBackup is free
+				File.Move(pathWant, pathBackup);
+			}
+			catch
+			{
+				// Eat it, this will happen rarely and the user will rarely need the file, so the odds of simply not making the backup is very unlikely
+				return false;
+			}
+
+			//hmm these might be asynchronous too
+			//wait for the move to complete, at least enough for pathWant to be cleared up
+			while (File.Exists(pathWant))
+				System.Threading.Thread.Sleep(10);
+
+			return true;
+		}
+
 		public static bool IsPowerOfTwo(int x)
 		{
 			if (x == 0 || x == 1)